vllm-project/vllm

[Transformers v5] IsaacForConditionalGeneration

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#38389 opened on Mar 27, 2026

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good first issuehelp wanted

Description

This is a sub-issue forming part of the work in https://github.com/vllm-project/vllm/issues/38379, please read the description of this issue before beginning to work on this one.

Which test is failing?

This one has lots of issues beyond this test, it should be upstreamed so that the code is correct and stays maintained

$ pytest tests/models/test_initialization.py::test_can_initialize_large_subset[IsaacForConditionalGeneration]
...
AttributeError: 'str' object has no attribute '__args__'

Related tests:

[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_single_image_models[isaac-test_case35] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)
[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_single_image_models[isaac-test_case36] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)
[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_single_image_models[isaac-test_case37] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)
[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_multi_image_models[isaac-test_case35] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)
[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_multi_image_models[isaac-test_case36] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)
[2026-03-27T01:46:05Z] FAILED models/multimodal/generation/test_common.py::test_multi_image_models[isaac-test_case37] - ImportError: cannot import name 'SlidingWindowCache' from 'transformers.cache_utils' (/usr/local/lib/python3.12/dist-packages/transformers/cache_utils.py)

How to configure my environment?

It's very important that you install both vLLM and Transformers from source so that your test results reflect the current state of both libraries.

# Or your fork
git clone https://github.com/huggingface/transformers.git
git clone https://github.com/vllm-project/vllm.git

cd vllm
VLLM_USE_PRECOMPILED=1 uv pip install -e .
uv pip install -e ../transformers

Contributor guide

Tech stack
python
Domain
testingbackend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
PythonHugging Face TransformerspytestvLLM codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Investigate the AttributeError and ImportError in tests/models/test initialization.py and tests/multimodal/generation/test common.py. The core issue is that 'SlidingWindowCache' cannot be imported from 'transformers.cache utils'. Check if this class exists in the transformers source and if the vLLM code expects an outdated version. Also refer to the parent issue #38379 for the broader upstreaming plan.